Narrow bandwidth has been one of the inherent major limitations of microstrip antennas where precise input voltage standing wave ratio (VSWR), maximum possible power gain, radiation pattern, and polarization characteristics have to be maintained over a wide operating temperature range. Changes in the antenna operating temperature affect the resonant frequency through thermal expansion, but primarily through changes in the substrate dielectric constant. The resonant frequency change can become comparable to the operating bandwidth, thus degrading the VSWR, gain, and other antenna parameters.
The resonant frequency of a radiating structure built on a Teflon-based substrate tends to increase with increasing temperature, as is well known in the art, due to thermal expansion and the negative temperature coefficient of substrate permittivity. One method for changing the resonant frequency of microstrip patch antennas is to use high-Q gallium arsenide (GaAs) varactor diodes connected to the radiating edges of the structure. This is an active temperature compensation scheme requiring an external power supply to bias the varactors and hence vary their capacitance. However, substrate size must be increased in order to accommodate the addition of these discrete GaAs components. In addition, GaAs varactor diodes are relatively expensive.
Accordingly, a need arises for a passive temperature compensation scheme that is relatively inexpensive to implement and does not have the size disadvantages of other approaches.